Microporous material is characterized by a large specific surface area in pores with a pore radius below 20 Å and is used in a large number of applications of considerable commercial importance. In most of these applications, the fact that the phase interface between the solid porous material and the medium (liquid or gas) in which it is used is large is of decisive importance. Heterogeneous phase catalysts for refinery processes for petrochemical conversion processes and for different environmentally related applications are often based on microporous material. Adsorbents for selective adsorption in the gas or liquid phase are microporous materials, like most of the inorganic ion exchangers used for selective separation of ionic compounds. In addition to these large scale and relatively established applications, microporous materials have recently become increasingly interesting in a number of more technologically advanced areas. Examples include use in chemical sensors, in fuel cells and batteries, in membranes for separation or catalytic purposes, during chromatography for preparative or analytical purposes, in electronics and optics and in the production of different types of composites.
Although a large phase interface is often a fundamental requirement for use of porous materials in different applications, a number of additional requirements related to the specific area of application are imposed on these materials. The large phase interface available in the micropores must be accessible and useable. Porosity, pore size and pore size distribution in large pores (meso- and macropores) are therefore often of major significance, especially when mass transport can be a rate-limiting factor. The chemical properties on the surface of the porous material can also be of decisive importance for the performance of the material in a given application. In this context, the purity of the material is, consequently, also significant. In most practical applications, the size and shape of the porous macrostructures and the degree of variation of these properties are of decisive importance. During use, the size and shape can influence properties like mass transport within the porous structures, pressure drop over a bed of particles of the material and the mechanical and thermal strength of the material. Which factor or factors are most important will vary strongly between different applications and are also highly dependent on the layout of the process in which the application occurs. Techniques that permit production of a material with increased specific surface area, pore structure (pore size/pore size distribution), chemical composition, mechanical and thermal strength, as well as increased and uniform size and shape, are consequently required to tailor porous inorganic macrostructures to different applications.
Microporous materials can be divided into crystalline molecular sieves and amorphous materials. Molecular sieves are characterized by the fact that they have a pore system through their regular crystal structure, in which the pores have a very well defined size in the range 2-20 Å with an exact value determined by the structure. The size of most molecules that are gases and liquids at room temperature, both inorganic and organic, is found within this size range. By selecting a molecular sieve with the appropriate pore size, use of molecular sieves for separation of one substance (one type of molecule) in a mixture is made possible by selective adsorption, hence the name molecular sieve. In addition to selective adsorption of uncharged substances, the well-defined micropore system of the molecular sieve offers a possibility for selective ion exchange of charged species and size-selective catalysis. In this case properties other than the micropore structure in molecular sieves are also of major significance, like ion exchange capacity or specific surface area and acidity. Molecular sieves can be divided into a number of subgroups, depending on chemical composition and structure. A commercially important subgroup are the zeolites, which, by definition, are crystalline microporous aluminosilicates. Another interesting subgroup is the microporous metal silicates, which are structural analogs of the zeolites, but do not contain any (or very little) aluminum.
A summary of the prior art, in terms of production, modification and characterization of molecular sieves, is described in the book Molecular Sieves—Principles of Synthesis and Identification (R. Szostak, Blackie Academic & Professional, London, 1998, Second Edition). In addition to molecular sieves, amorphous microporous materials, chiefly silica, aluminum silicate and aluminum oxide, have been used as adsorbents and catalyst supports. A number of long-known techniques, like spray drying, prilling, pelletizing and extrusion, have been and are being used to produce macrostructures in the form of, for example, spherical particles, extrudates, pellets and tablets of both micropores and other types of porous materials for use in catalysis, adsorption and ion exchange. A summary of these techniques is described in Catalyst Manufacture, A. B. Stiles and T. A. Koch, Marcel Dekker, New York, 1995.
Because of limited possibilities with the known technique, considerable investment has been made to find new ways to produce macrostructures of microporous materials, with a certain emphasis on those in the form of films.
EP 94/01301 describes-production of films of molecular sieves by a process in which seed crystals of molecular sieves are deposited on a substrate surface and then made to grow together into a continuous film. GB 94/00878 describes production of films of molecular sieves by introduction of a substrate to a synthesis solution adjusted for zeolite crystallization and crystallization with a gradual increase in synthesis temperature. SE 93/00715 describes production of colloidal suspensions of identical microparticles of molecular sieves with an average size below 200 nm. SE 90/00088 describes a method for production of an adsorbent material in the form of a monolith by impregnation of the monolithic cell structure with a hydrophobic molecular sieve, followed by partial sintering of the molecular sieve with the material from which the cell structure is constructed.
Although a number of different techniques already exist for production of microporous inorganic macrostructures with the desired size and shape, these techniques are beset with a number of limitations that affect the properties of the macrostructures during use in the intended application. Most of these techniques require the use of a binder to give the structure acceptable mechanical strength. This binder often adversely affects other desired properties, like high specific surface area and uniform chemical composition. For most of the existing techniques, other possibilities for keeping variations in size and shape within narrow limits are sharply constrained. If a well defined size is desired with a narrow particle size distribution, one is most often obliged to carry out processing by separation, which leads to considerable waste during manufacture. The use of different types of binders also affects the pore structure in the resulting macrostructure and it is often necessary to find a compromise, in which the desired pore structure is weighed against the mechanical properties of the material. It is often desirable to have a bimodal pore size distribution in the macrostructures of macroporous materials, in which the micropores maintain a large specific phase interface, whereas the larger pores in the meso- or macropore range permit transport of molecules to the surface and, in this way, prevent limitations caused by slow diffusion. During production of microporous macrostructures according to the known technique, a secondary system of pores within the meso- and/or macropore range can be produced by admixing a particulate inorganic material or by admixing organic material (for example, cellulose fibers), which are later eliminated by calcining. Both of these techniques, however, produce an adverse effect on the other properties of the resulting material.